A liquid crystal cell of a liquid crystal display device essentially consists of a thin film transistor (TFT) substrate for driving the liquid crystal display device, a color filter for color display, and a liquid crystal layer between the two substrates. The color filter is produced by subjecting a photosensitive organic material, in which at least one pigment is dispersed, to photolithography to form a pattern and coating three or more kinds of color inks having different transmission-absorption wavelengths on the pattern to form pixels. The pixels are combined to create color images. If needed, overcoats are formed on the pixels of the color filter substrate to reduce the step height between the adjacent pixels, or column spacers are formed by patterning to maintain an inner gap of the liquid crystal cell at a constant level.
Photoresist compositions, particularly, negative type compositions patternable by photolithography are generally used to form the overcoats or the column spacers. A typical negative type photoresist composition is a mixture composed essentially of an alkali-soluble polymer, a polyfunctional monomer containing two or more acrylate groups, and a photoinitiator. If necessary, the negative type photoresist composition may further comprise a solvent and one or more additives selected from surfactants, adhesion aids, etc. The photoinitiator is decomposed to generate active radicals when the negative type photoresist composition is exposed to light, particularly UV. The active radicals activate the acrylate groups of the polyfunctional monomer to induce crosslinking of the alkali-soluble polymer with the polyfunctional monomer (photopolymerization). This photopolymerization increases the molecular weight of the polymer in the crosslinked portion (i.e. in the portion exposed to light), and as a result, the exposed portion becomes insoluble in an alkaline solution. The exposed portion remains unremoved after subsequent development. Consequently, the use of the negative type photosensitive resin enables the formation of a fine pattern by photolithography. The sensitivity of the photoresist composition is associated with the minimum exposure dose (i.e. light energy) at which the pattern is stably formed. A low exposure dose shortens the processing time, contributing to the improvement in productivity. Particularly, the sensitivity of a photoresist composition for forming column spacers is determined by an exposure dose at which the variation in the thickness of a pattern is sharply decreased.
Photoinitiators that rapidly respond to low light energy have been used to improve the sensitivity of photoresist compositions without any marked reduction in the developability of the photoresist compositions. However, the use of photoinitiators is disadvantageous in that the variation in the thickness of patterns depending on the exposure dose is small. This disadvantage makes it difficult to form transparent thin films whose thickness is controlled by varying the slit structure and transmittance of photomasks.